Previous studies provided strong evidence for genetic factors controlling insulin action and insulin release in Pima Indians. Both, impaired insulin action (insulin resistance) in the skeletal muscle, and a defective glucose-stimulated insulin release appear to have a primary role in the development of NIDDM in this population. Initial analyses indicated a linkage of insulin action with the MNS blood group locus on the long arm of chromosome 4 (4q). We have expanded this observation by including several DNA-based polymorphic markers spanning approximately 133 map units (centiMorgans, cM) around MNS. The results were consistent with the presence of a putative gene on 4q controlling insulin action. Two markers separated only by 1 cM, the annexin V gene (ANX5) and the intestinal fatty acid binding protein gene (FABP2) showed the strongest linkage with the insulin resistance phenotype. Extensive typings of all currently known DNA markers in this region narrowed the chromosomal segment harboring the putative gene to about 5 cM encompassing ANX5 and FABP2. Because the only known functional genes in this region, ANX5 and FABP2, are unlikely candidates, we have started a long-range genomic cloning using the yeast artificial chromosome (YAC) approach to attempt the isolation of the putative gene. Our strategy includes isolation of overlapping YAC clones spanning the chromosomal segment of interest, and identification of novel highly polymorphic DNA markers which will be typed in individuals involved in this study. Clone(s) identified by markers showing the most significant linkage with the insulin resistance will be subject to the search for the candidate gene.